Photolyase
Encyclopedia
Photolyases are DNA repair
enzyme
s that repair damage caused by exposure to ultraviolet
light. This enzyme mechanism requires visible light, preferentially from the violet/blue end of the spectrum, and is known as photoreactivation.
Photolyase is a phylogenetically
old enzyme which is present and functional in many species, from the bacteria
to the fungi to the animal
s. However it is no longer working in humans and other placental mammals who instead rely on the less efficient nucleotide excision repair mechanism
.
Photolyases bind complementary DNA
strands and break certain types of pyrimidine dimers
that arise when a pair of thymine
or cytosine
bases on the same strand of DNA become covalently
linked. These dimers result in a 'bulge' of the DNA structure, referred to as a lesion. The more common covalent linkage involves the formation of a cyclobutane
bridge. Photolyases have a high affinity for these lesions and reversibly bind and convert them back to the original bases.
Photolyases are flavoprotein
s and contain two light-harvesting cofactor
s. All photolyases contain the two-electron-reduced FADH-
; they are divided into two main classes based on the second cofactor, which may be either the pterin
methenyltetrahydrofolate (MTHF) in folate photolyases or the deazaflavin 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF) in deazaflavin photolyases. Although only FAD is required for catalytic activity, the second cofactor significantly accelerates reaction rate in low-light conditions. The enzyme acts by electron transfer
in which the reduced flavin FADH- is activated by light energy and acts as an electron donor to break the pyrimidine dimer.
On the basis of sequence similarities DNA photolyases can be grouped into two classes. The first class contains enzymes from Gram-negative and Gram-positive bacteria, the halophilic archaebacteria Halobacterium halobium, fungi and plants. Proteins containing this domain also include Arabidopsis thaliana
cryptochrome
s 1 and 2, which are blue light photoreceptors that mediate blue light-induced gene expression and modulation of circadian rhythm
s.
Some sunscreen
s include photolyase in their ingredients, claiming a reparative action on UV
-damaged skin.
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s that repair damage caused by exposure to ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light. This enzyme mechanism requires visible light, preferentially from the violet/blue end of the spectrum, and is known as photoreactivation.
Photolyase is a phylogenetically
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
old enzyme which is present and functional in many species, from the bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
to the fungi to the animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s. However it is no longer working in humans and other placental mammals who instead rely on the less efficient nucleotide excision repair mechanism
Nucleotide excision repair
Nucleotide excision repair is a DNA repair mechanism. DNA constantly requires repair due to damage that can occur to bases from a vast variety of sources including chemicals, radiation and other mutagens...
.
Photolyases bind complementary DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
strands and break certain types of pyrimidine dimers
Pyrimidine dimers
Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions. Ultraviolet light induces the formation of covalent linkages by reactions localized on the C=C double bonds. In dsRNA, uracil dimers may also accumulate as a result of UV radiation...
that arise when a pair of thymine
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
or cytosine
Cytosine
Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
bases on the same strand of DNA become covalently
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
linked. These dimers result in a 'bulge' of the DNA structure, referred to as a lesion. The more common covalent linkage involves the formation of a cyclobutane
Cyclobutane
Cyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
bridge. Photolyases have a high affinity for these lesions and reversibly bind and convert them back to the original bases.
Flavoprotein
Flavoproteins are proteins that contain a nucleic acid derivative of riboflavin: the flavin adenine dinucleotide or flavin mononucleotide ....
s and contain two light-harvesting cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....
s. All photolyases contain the two-electron-reduced FADH-
FAD
In biochemistry, flavin adenine dinucleotide is a redox cofactor involved in several important reactions in metabolism. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin moiety bound to the phosphate...
; they are divided into two main classes based on the second cofactor, which may be either the pterin
Pterin
Pterin is a heterocyclic compound composed of a pteridine ring system, with a keto group and an amino group on positions 4 and 2 respectively...
methenyltetrahydrofolate (MTHF) in folate photolyases or the deazaflavin 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF) in deazaflavin photolyases. Although only FAD is required for catalytic activity, the second cofactor significantly accelerates reaction rate in low-light conditions. The enzyme acts by electron transfer
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
in which the reduced flavin FADH- is activated by light energy and acts as an electron donor to break the pyrimidine dimer.
On the basis of sequence similarities DNA photolyases can be grouped into two classes. The first class contains enzymes from Gram-negative and Gram-positive bacteria, the halophilic archaebacteria Halobacterium halobium, fungi and plants. Proteins containing this domain also include Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
cryptochrome
Cryptochrome
Cryptochromes are a class of blue light-sensitive flavoproteins found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species...
s 1 and 2, which are blue light photoreceptors that mediate blue light-induced gene expression and modulation of circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
s.
Some sunscreen
Sunscreen
Sunblock is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn...
s include photolyase in their ingredients, claiming a reparative action on UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
-damaged skin.